Choosing Direct Hot Air or Hydronic Marine Heating

Direct Hot Air or Hydronic Marine Heating

Before you embark on a project to heat your boat, you should think about whether you want to use a direct air heater or a hydronic system. Each has advantages and disadvantages that should be considered.

Direct Air Heating

A direct air heating system consists of a diesel air heater that generates hot air directly, which is then ducted around the boat. We offer diesel air heater kits at different sizes and price points to suit all boats. You can read more about this specific option here.

Hydronic Air Heating

Hydronic air heating runs off a diesel furnace that heats coolant, which is circulated in the boat to fan heads. Hydronic systems can include water heating as well. 

Hydronic Systems


  • Hydronic systems can provide hot air and hot water in one system. This facilitates hot water when engine heat is not available, which can be a real advantage.
  • Existing hot water cylinders can often be incorporated into the system. For example, using the Dieselheat DH1.
  • Hot coolant piping can be easier to run in a boat than ducting (it has a smaller diameter so will wiggle through tight spaces or can sit in the bilge space).
  • Getting outlets in hard to reach spots can be achievable.
  • A combined hydronic and hot water system is usually more compact than separate systems.


  • Heat modulation (controlling the air temperature) can be trickier unless specific thermostats are installed. 
  • Systems can take longer to get started and warm up, especially when heating water as well.
  • Systems generally require a bit more fine tuning or balancing to get them working properly.
  • Hydronic fan heads do not project the hot air as forcefully as a direct diesel heater, so the hot air does not spread out as much in the boat.
  • We use the quietest Kalori fan heads available, however, in some cases they are slightly noisier than ducted air heating because the fans are at the point of heating in the living space.

Direct Air Heating


  • Can be the quickest and cheapest systems to install/set up.
  • Has the fastest heat up time.
  • Outlet air velocity from hot air vents is faster, so the hot air tends to spread out better in the vicinity of the outlet.
  • Temperature control is easier as outputs are more variable, air heaters modulate their output better than a fan head.


  • Does not resolve lack of hot water issues.
  • Larger diameter ducting takes more space and loses more heat than hydronic pipes.
  • Getting air distributed throughout the boat can be harder due to the ducting sizes.


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